Units of a telecommunication system, such as a master unit in a distributed antenna system (“DAS”), may include interface cards for communicating signals between units of the DAS and base stations and/or repeaters in communication with the DAS. Interface cards of a master unit may include direct conversion transmitter or other suitable transmitting devices for transmitting uplink signals from a master unit to a base station or repeater. Direct conversion transmitters can generate uplink signals with a carrier signal component at a center frequency and signal components at frequencies above and below the carrier frequency, such as sideband signal components, that can be modulated to communicate information via the uplink signal. Direct conversion transmission may involve information being communicated by modulating the sideband signal components having relatively higher power than the carrier signal.
Direct conversion transmitters can include local oscillators, mixers, summers, and amplifiers. The local oscillators can generate radio frequency (“RF”) signals at an output frequency for a transmitter of the unit. The RF signals from local oscillators can be combined with modulating signals having information to be transmitted via mixers to create a modulated RF signal. Amplifiers can increase the amplitude or strength of signals. Low frequency amplifiers may be used to amplify the modulating signals. High frequency RF amplifiers may be used to amplify modulated radio signals. Summing circuits may add or otherwise combine two signals to generate a sum signal.
One disadvantage of using direct conversion transmitters is the generation of undesirable signal components such as local oscillator leakage signals. A local oscillator leakage signal can be a signal transmitted by a direct conversion transmitter at a carrier frequency. The signals provided by local oscillators of a direct conversion transmitter may have a frequency at or near the carrier frequency. Normal operation of a direct conversion transmitter may include minimizing the power of the signal components provided by local oscillators such that sideband signal components having data to be transmitted have more power than the local oscillators. However, errors in local oscillator circuits may cause the transmitter to emit power at or near the carrier frequency. The undesired signal component transmitted at the carrier frequency is a local oscillator leakage signal that can reduce the performance of the transmitter.
Another disadvantage of using direct conversion transmitters is the generation of undesirable sideband image signals. Sideband image signals can be caused by gain imbalance and errors in a quadrature-mixing phase difference between an in-phase (“I”) component and a quadrature (“Q”) component of a complex uplink signal. A gain imbalance can be caused by an uplink signal having an I component and a Q component with different signal powers. A quadrature-mixing phase difference error can be caused by oscillators of an I/Q modulator or demodulator using signals that do not differ by exactly ninety degrees. Both gain imbalance and quadrature-mixing phase errors can generate sideband images of sideband signal components. I/Q gain imbalance and I/Q phase difference errors in quadrature mixing can cause the power of sideband image signal components to exceed a spectral mask for a telecommunication system.
It is desirable to reduce local oscillator leakage power and sideband image power in telecommunication systems.